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Microbiology and Macrofaunal Activity in Soil beneath Shrub Canopies during Residue Decomposition in Agroecosystems of the Sahel
A major limitation for crop productivity in degraded Sahelian soils is the lack of organic inputs. Piliostigma reticulatum (DC.) Hochst and Guiera senegalensis J. F. Gmel. are unrecognized and unmanaged shrubs of the Sahel that coexist with crops and have potential to provide significant amounts of organic inputs to improve soil quality. However, conventional management involves coppicing and burning aboveground biomass every spring before crop plantings. Therefore, the objective of this study was to develop fundamental microbial information about nonthermal decomposition of these residues as influenced by the shrub canopy, litter, and root system. An experiment was done for P. reticulatum or G. senegalensis that had a 2 by 3 by 2 factorial design with two soil treatments (beneath and outside the influence of the shrub), three residue amendments (leaf, stem, and leaf + stem), and two litterbag mesh size treatments (plus, >2 mm, or minus macrofauna). Litterbags were destructively sampled at 15, 30, 60, 120, and 210 d after the first rain. At each sampling, litter mass was determined, and the soil beneath the litter bag was assessed for microbial properties and inorganic N. The presence of macrofauna stimulated mass loss, microbial biomass carbon (MBC), enzyme activities, and inorganic N. Rates of decomposition and microbial response with soils beneath were higher than outside the canopy. The results provide a basis for developing improved, non-thermal management of coppiced shrub residue while still allowing preparation of favorable seedbeds for row crops. Previous research has shown that these shrubs can do hydraulic redistribution (HR) of water from wet subsoil to dry surface soil. The higher soil moisture beneath shrubs in our experiment indicates that HR is a contributing factor to drive decomposition and nutrient mineralization in the dry season.
Crop Productivity and Nutrient Dynamics in a Shrub-Based Farming System of the Sahel
Piliostigma reticulatum (DC.) Hochst., an indigenous shrub, forms an important vegetative component of parkland cropping systems in the Sahel; however, its biophysical interactions with soil and crops are not well understood. Therefore, the objectives were to determine the impact of P. reticulatum, under varying fertilizer rates, on crop yield response and soil nutrient dynamics. The experiment had a split-plot factorial design, where the main plot was shrub (presence or absence) and the subplot was fertilizer rate (0, 0.5, 1.0, or 1.5 times the recommended N–P–K fertilizer rate) applied to a peanut (Arachis hypogaea L.)–pearl millet [Pennisetum glaucum (L.) R. Br.] rotation. In 3 of the 4 yr, P. reticulatum improved or had no effect on crop yields when averaged across fertilizer rates. Overall, millet and peanut biomass and N and P uptake by millet increased in the presence of shrubs and with increasing fertilizer rate. Contrary to P, inorganic N in the soils changed very rapidly, reaching very low levels by the end of the growing season. The N content of soil leachates below the rooting zone was generally lower beneath than outside the shrub canopy, suggesting that the shrub conserves N that is otherwise lost through leaching. Piliostigma reticulatum increased particulate organic matter, indicating that this shrub improved soil quality. These results suggest that P. reticulatum, under nonthermal management and a higher density than typically found in farmers’ fields, has ecological benefits with improved soils and reduced loss of N, which has implications throughout the Sahel.
Insecticide Residues in Soil and Water in Coastal Areas of Vegetable Production in Togo
Some common organochlorine, organophosphorus and pyrethroid insecticides were analysed in agricultural soil samples (n=35) and surface water and groundwater samples (n=25) collected from coastal areas of vegetable production in Togo. Analytical methods included solvent extraction of the insecticide residues and their subsequent quantification using GCECD. δ-HCH, heptachlor epoxide, 4,4-DDE, endosulphan (α, β and sulphate), lambda-cyalothrin and chlorpyrifos were found in the soil samples with concentrations that varied from non-detectable (ND) to 26.93 μg kg−1 dry weight. For water samples, heptachlor epoxide, 2,4-DDD, 4,4-DDD, 4,4-DDE and endosulphan (α, β, and sulphate) were found at contamination levels that varied from ND to 0.116 μg L−1 . The concentration of insecticide residues detected in the water samples was below the limits set by the World Health Organization (WHO) and also by the European Union (EU), with the exception of the concentration of endosulphan sulphate at the Aného site, which was 0.116 μg L−1 .
An Assessment of Inherent Chemical Properties of Soils for Balanced Fertilizer Recommendations for Cocoa in Ghana
Sustainable cocoa production in Ghana would require a shift in fertilizer recommendations from general applications to site-specific recommendations of fertilizers that account for initial fertility status and actual nutrient needs of soils on which cocoa is grown. A soil fertility survey was conducted in the major cocoa regions of Ghana covering the major benchmark soils. Two hundred and twenty four plots were sampled and composite surface soils collected and analyzed for selected fertility characteristics. The results show that most of the cocoa soils have low inherent fertility characterized by low C, N and exchange capacity. All the cocoa soils sorb P, which may limit availability of P in the soil solution. The soils generally are acidic, and soils in Western region, especially the Ferralsols, show the most acidic reaction with substantially measurable exchangeable Al. The results suggest that these differential characteristics of the surveyed soils should be considered in formulating balanced site-specific fertilizer for cocoa in Ghana.
Improving Fertilizer Recommendations for Cocoa in Ghana Based on Inherent Soil Fertility Characteristics
In Ghana, cocoa has traditionally been grown as a low input crop, which has caused soil fertility deterioration, and thus, the need to integrate fertilizer use into cocoa agricultural practices. However, fertilizers recommended to farmers are general in nature and do not account for specific crop needs and inherent soil fertility conditions. This study evaluates the use of a soil diagnosis model to determine fertilizer recommendations for cocoa based on inherent soil fertility characteristics in the cocoa growing zones of Ghana. The site-specific fertilizer formulations were tested against blanket recommendations (Asaase Wura and Cocofeed) in farmers’ settings from 2009 to 2011. The results showed that DS-formulated site-specific fertilizer performed better than all blanket fertilizers in Western soils especially on the Ferralsols which are very acidic and depleted of base cations. On the other soil conditions, the site-specific formulations were comparable to the blanket formulations. Trend analysis of cocoa response to applied fertilizer suggests that P is a major determinant of cocoa productivity and that P2O5 rates >120 kg ha−1 would be required, when justified economically, for optimal cocoa yield, while potassium could be kept at around 45 kg K2O ha−1. In view of these results, the cocoa fertilizer formulas proposed for western regions of Ghana could be revised according to the DS model recommendations by taking into consideration the optima presented above. For the other cocoa regions, the DS would not be economic and therefore, proposed formulas should keep P2O5 and K2O around the optima above-presented while compensating for nutrients exported by the crop.